1. Field of the Invention
This invention generally relates to an inductive device including a winding on a magnetic core. This invention specifically relates to a line filter provided in, for example, a power supply circuit of an electronic apparatus to prevent entrance and leakage of noise into and from the electronic apparatus via a power supply line.
2. Description of the Prior Art
A typical prior-art line filter is used to damp or suppress common-mode noise (generally, radio-frequency noise) which occurs between a power supply line and a ground. The line filter includes a pair of windings on a magnetic core. The filter windings are disposed in a pair of line segments of the power supply line respectively. A line current passing through the filter windings causes magnetic flux. The magnetic core has such a configuration as to form a closed magnetic circuit for the magnetic flux. To prevent the magnetic core from being magnetically saturated, the combination of the magnetic core and the filter windings is designed so that the magnetic flux generated by one of the windings can cancel the magnetic flux generated by the other winding.
A first example of such a prior-art line filter includes a magnetic core with a single opening, and a bobbin made of resin which is positioned around one leg of the magnetic core. A pair of windings are provided on the bobbin. The bobbin is formed with a gear. During assembly of the line filter, the bobbin is rotatably supported on the leg of the magnetic core, and ends of wires are connected to the bobbin. A suitable drive mechanism is set into engagement with the gear on the bobbin. As the bobbin is rotated relative to the magnetic core by the drive mechanism, the wires are automatically wound on the bobbin to provide the filter windings. After the filter windings have been completed, the bobbin and the magnetic core are bonded together by adhesive.
The adhesive tends to apply a stress to the magnetic core. Such a stress causes a reduction of inductances of the filter windings by 5-30% as the line filter ages.
A second example of the prior-art line filter includes a magnetic core with two openings, and a bobbin made of resin which is positioned around a central leg of the magnetic core. A pair of windings are provided on the bobbin. The bobbin is formed with a gear. The second example of the prior-art line filter is assembled similarly to the assembly of the first example of the prior-art line filter.